1. Field of the Invention
This invention relates to the use of ethylene oxide/propylene oxide block copolymers obtained by addition of ethylene oxide onto polypropylene glycol and having a content of 5 to 35% by weight, based on the total weight of the block copolymers, of ethyleneoxy groups and a number average molecular weight of the polypropylene glycol of 1,500 to 3,000 as an additive for preventing dust emission from dry, hydraulically setting mixtures, more particularly cement-based primers.
In the context of the invention, hydraulically setting mixtures are mixtures, more particularly based on gypsum and/or cement, which are used for building purposes. In the context of the invention, cement-based primers also include levelling, equalizing and repair mixtures. The mixtures in question are fine-particle powders and, in the interests of simplicity, are collectively referred to hereinafter as primers.
2. Statement of Related Art
Primers are used inter alia for the professional preparation of substrates for laying floor coverings. They contain cement, for example Portland cement and/or highalumina cement, gypsum, quartz sand, limestone powder and other inorganic fillers and also inorganic and organic additives, such as cellulose derivatives, redispersion powders, casein, vegetable proteins, more particularly wheat protein, and the like.
Primers are marketed as fine powders which are mixed with water at the building site. Flat, smooth, sufficiently absorbent and firm substrates, for example for floor coverings, can be formed with the water-based mixtures. Stirring of the powders with water is accompanied by serious dust emission. Since cement is a skin irritant on account of its high alkalinity, affecting mucous membrane in particular, such dust emission is undesirable.
The emission of dust from hydraulically setting mixtures, particularly primers, is normally reduced through the degree of grinding or through the particle composition of the powder-form product. However, this adversely affects the processability of relatively coarse powders. Another known method is the aggregation, for example with water, aqueous solutions or dispersions. Thus, it is known from U.S. Pat. No. 4,780,143 that water-based foam can be added to clinker before it is ground to cement in order to reduce dust emission. According to JP-A 63/2847, synthetic resin dispersions are added to cement for air-placed concrete compositions in order to reduce dust emission.
Temporary aggregation in hydraulically setting mixtures is of no significance when the products are to be subsequently ground or used as air-placed concrete or air-placed mortar. However, in fine-particle primers, on which floor coverings are to be laid after application, relatively coarse aggregations are unacceptable because they show up on the surface of many smooth floor covering materials.
It is also known that dust reducing agents can be added to water used to mix cement in order to reduce dust emission during spraying. Thus, JP-A 61/31335 describes mixing water additives, such as polyethylene glycol, for air-placed plaster while JP-A 57/149856 describes polyethylene glycol as an additive for gypsum mixing water to reduce subsequent dust emission during cutting of the gypsum products. JP-A 58/15056 and 59/141448 describe the addition of polyethylene glycol with an average molecular weight of 2000 to dry air-placed concrete mixes for reducing dust emission. Finally, it is mentioned in a company publication (Erbsloh, Dusseldorf) entitled Das Pluronic-Gitternetz, Edition IV, that ethylene oxide/propylene oxide block copolymers can be used as a dust-binding agent although there is no reference to the dust-emitting materials suitable for treatment with these mixtures, which encompass a number of products having various contents of ethyleneoxy groups, molecular weights, melting and solidification points and the like.
However, it has been found that polyethylene glycol, glycerol and a large number of the ethylene oxide/propylene oxide block copolymers mentioned in the above-cited company publication Das Pluronic-Gitternetz undesirably delay the setting of primers and make the primers hygroscopic, so that they are unsuitable for the application with which the present invention is concerned. If, on the other hand, hydrophobic additives, including a large number of ethylene oxide/propylene oxide block copolymers known from the above cited company publication, are used to reduce dust emission, difficulties arise in regard to the wetting of the powder-form primers.